In recent years, personal communication devices, such as mobile phones, have been remarkably improved. For personal communication devices, high frequency analog devices became more important than ever. For manufacturing a personal communication device to be small in size, surface-acoustic-wave (SAW) devices are useful. For example, surface-acoustic-wave devices are used for filters, resonator and the like.
A conventional surface-acoustic-wave (SAW) device includes a transducer, which converts an electric signal into acoustic waves or converts acoustic waves into an electric signal. Such a transducer may be made of a piezoelectric material. The piezoelectric material produces a mechanical distortion in response to application of voltage. In reverse, the piezoelectric material generates electric polarization as a result of the application of mechanical stress. Usually, as piezoelectric crystal, quartz, lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3) is used.
A conventional SAW transversal filter includes a piezoelectric substrate and IDTs (Interdigital Transducers). When an input signal is applied to one IDT, a mechanical distrotion is generated with a certain frequency and surface-acoustic-waves are generated on the surface of the piezoelectric substrate. The surface-acoustic waves travel on the surface of the piezoelectric substrate to another IDT, and aer converted into an electrical signal.
The surface-acoustic-wave device can be mounted on a base substrate in the same manner as an LSI, using wire-bonding technique. In packaging process, it is required to have an air gap around the functional surface of the piezoelectric substrate. For example, the piezoelectric substrate is mounted in a cavity of a base substrate, and a seal lid is provided to make the air gap above the piezoelectric substrate.
According to the above described conventional technology, it is difficult to package a surface-acoustic-wave device in a small space with lower cost.